Evolving radio communications technologies create mobile communications environments having variable sets of radio access technologies (RATs). For example, in a given geographical area, the following RATs may be available: Wideband Code Division Multiple Access (WCDMA), Universal Terrestrial Radio Access Network (UTRAN) and Evolved UTRAN (E-UTRAN). In another geographical area, a different set of RATs may be available; for example, only Global System for Mobile communications (GSM) may be available. A mobile terminal user would want a mobile terminal capable of operating in both areas. Thus, mobile terminal manufacturers offer mobile terminals (a.k.a., user equipments (UEs)) capable of operating with multiple RATs (i.e., multi-mode UEs).
Users also desire seamless UE mobility within and between geographic areas. Thus, manufacturers of UEs face the further demand for multi-mode UEs with a high degree of RAT interoperability to support transparent mobility. For example, given the mobile communications environment discussed in the preceding paragraph, a user may wish to initiate a call in the geographic area covered by GSM and have full call continuity as the UE travels from that area to the area covered by E-UTRAN. Thus, the UE must be capable of monitoring channels in both RATs while a call is in progress.
Fortunately, E-UTRAN, often referred to as Long Term Evolution (LTE), provides for such multi-RAT connected mode mobility. However, LTE imposes a burden on UEs to monitor up to three layers (i.e., non-serving frequency carriers), as instructed by the network, for each RAT supported, regardless of the number of RATs that are supported by the UE. On each layer the UE is required to monitor multiple cells e.g. between 4-6 cells per layers depending upon the type of RAT. Thus, LTE requires a multi-mode UE capable of operating with, for example, four RATs (e.g. multi-mode UE supporting LTE FDD, LTE TDD, UTRAN FDD and UTRAN TDD), to monitor up to twelve layers in parallel in a mobile communications environment having those four RATs. Assuming 5 cells to be monitored per layers, such a UE will have to monitor in total 60 cells in parallel (i.e., using the same gap pattern.) Parallel monitoring implies that UE monitors all the requested layers during a single periodic gap. In LTE, gap patterns with two different periodicity are define: 6 ms gap occurring every 40 or 6 ms gap occurring every 80 ms. Only one of them can be activated at a time. The LTE layer monitoring requirements means that a UE manufacturer must weigh considerable cost/benefit tradeoffs for each additional RAT supported by the UE as each additional supported RAT means that additional memory, radio, and computational resources are required to accommodate the increased layer monitoring requirements. Thus, it is desirable for the UE to have some control over layer monitoring requirements imposed by the network.